International Journal of the Physical Sciences Vol. 6(17), pp. 4287-4293, 2 September, 2011 Available online at http://www.academicjournals.org/IJPS ISSN 1992 - 1950 ©2011 Academic Journals

Full Length Research Paper

Controlling crystallization and morphologies of monoclinic bismuth vanadate (BiVO4) dendrite with

enhanced photocatalytic activities

Haoyong Yin1, Yongfei Sun1, Dejun Shi1, Xiaoxi Wang1 and Ling Wang2*

1College of Materials and Environmental Engneering, Hangzhou Dianzi university, Hangzhou, 310018, P. R. China.

2Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou, 310018, P. R. China.

Accepted 14 July 2011

The monoclinic BiVO4 dendrites constructed by Bismuth vanadate (BiVO4) branches with average diameters of about 100 nm have been successfully fabricated by Sodium dodecyl sulfate (SDS) assistant hydrothermal process. The monoclinic BiVO4 dendrites were characterized by X-ray diffraction, scanning electron microscopy, Raman spectroscopy and transmission electron microscopy. The monoclinic BiVO4 dendrites have uniform branches and the branches were transformed from the aggregated monoclinic BiVO4 nanoparticles through the Ostwald ripening process. The monoclinic BiVO4 dendrites show high efficient photocatalytic activities in decomposition of methyl orange. Key words: Controlling crystallization, hierarchical structures, bismuth vanadate dendrites.

INTRODUCTION Nowadays, the controllable synthesis of micro- and nano-scale materials with unique morphology and hierarchy has stimulated intensive interest due to their importance in basic scientific research and technological applications (Wang et al., 2006; Xia et al., 2003; Huang et al., 2006). Many efforts have been focused on the assembly of lower dimensional nanostructures into three-dimensional ordered superstructures, such as multipods, snowflakes and dendritic structures (Toprak et al., 2007; Ding et al., 2006; Wu et al., 2006). However, due to the difficulties in controlling the nucleation and growth processes, the construction of well - defined three - dimensional architectures with lower dimensional components is still a great challenge (Whitesides et al., 2002; Park et al., 2004; Xu et al., 2008). These hierarchical structures may endow them with morphology-dependant properties and imply their potential application in various fields. Therefore, further research is warranted to realize the advantages of them. Bismuth vanadate (BiVO4), known for its ferroelasticity (Baker et al., 1991) ionic conductivity (Abraham et al., 1988) and pigmentation (Wood et al., *Corresponding author. E-mail: yhy@hdu.edu.cn. Tel: +86-591-87441126.

2004) has been extensively investigated recently. It has been used for a wide range of applications including gas sensors, posistors, solid-state electrolytes, positive electrode materials for lithium rechargeable batteries, and nontoxic yellow pigment for high-performance lead-free paints (Zhao et al., 2008) and recently proved to be a good photocatalyst for water splitting and pollutant decomposing under visible light irradiation (Ke et al., 2009; Shang et al., 2009). In the three structure types of bismuth vanadate–zircon tetragonal, scheelite monoclinic, and scheelite tetragonal, only the monoclinic scheelite BiVO4 exhibits much higher visible-light photocatalytic activity over the other forms, giving rise to more attentions and wider researches (Zhang et al., 2006; Su et al., 2010, 2009). Besides crystalline form, the photocatalytic property of BiVO4 also depends strongly on its microstructure, which is related to the synthesis method. To further improve the visible-light photocatalytic activity, a few submicron- or nanometer-sized BiVO4 with various morphologoies have been prepared. Spindle-like BiVO4 modified by polyaniline (PANI) (Shang et al., 2009) was synthesized via a sonochemical approach which shows efficient photocatalytic activity in the degradation of rhodamine (RhB) and phenol. Hydrothermal method was used to synthesize the bismuth vanadate (BiVO4) nanosheets with good visible photocatalytic activities

4288 Int. J. Phys. Sci. (Zhang et al., 2006). Pyramidal-shaped BiVO4 nanowire arrays (Su et al., 2010) were vertically oriented to a fluorine doped tin oxide coated glass substrate by seed-mediated growth approach and show higher photocurrents in photoelectrochemical water splitting. The starlike BiVO4 was also prepared using water/ethanol mixture as the solvent which exhibited high visible-light-driven photocatalytic efficiency (Sun et al., 2009). Considering the properties and applications are closely related to the microstructure and corresponding synthetic techniques and processes, it is of great importance to study the controllable synthesis of BiVO4 with the desired crystalline phase and morphology.

Herein, we reported the preparation of monoclinic BiVO4 dendrite by a simple hydrothermal method using SDS (Sodium dodecyl sulfate) to control its crystallization and morphologies. In this method SDS can both determine the crystalline phase and morphology of BiOV4. Encouragingly, the as prepared monoclinic BiVO4 dendrites exhibit high photocatalytic efficiency. MATERIALS AND METHODS

All chemicals were analytical grade and were used as received without further purification. In a typical synthesis, 50 ml 0.13 M NH4VO3 solution was prepared by dissolving 6.5 mmol NH4VO3 into 50 ml NH3•H2O solution (2 M). Meanwhile, 6.5 mmol BiCl3 was dissolved in 50 ml of HNO3 (4 M) which was added 5 mmol SDS in advance. Then NH4VO3 solution was slightly added into BiCl3 solution under magnetic stirring. At last, the pH was adjusted by NH3•H2O solution. This precursor solution was poured into a Teflon-lined stainless steel autoclave until 80% of the volume of the autoclave was occupied. The autoclave was heated at 160°C for 10 h at autogenous pressure. After the autoclave was cooled to room temperature, the precipitate was separated by filtration, washed with distilled water and absolute alcohol several times, and then dried at 60°C for 4 h. For comparison, another BiVO4 sample was also prepared by the same procedure without adding SDS. The powder X-ray diffraction (XRD) patterns of as-synthesized samples were measured on a X-ray diffractometer (Bruker D8 ADVANCE) using monochromatized Cu Kα (λ = 0.15418 nm) radiation under 40 kV and 100 mA. The morphologies and microstructures of as-prepared samples were examined with scanning electron microscopy (SEM, JSM-6700F). Transmission electron microscopy (TEM) observations were carried out on a JEOL JEM-2100 instrument with accelerating voltage 200 kV in bright-field. The specimens used for TEM studies were dispersed in absolute ethanol by ultrasonic treatment. The sample was then dropped onto a copper grid coated with a holey carbon film and dried in air. The Raman spectra were analyzed using a German Bruker RFS 100/S Raman spectrometer. Brunauer–Emmett–Teller (BET) surface area was determined by nitrogen adsorption using a Micromeritics ASAP 2000 system.

Photocatalytic activities of the monoclinic BiVO4 dendrites were evaluated by degradation of methyl orange under visible-light irradiation of a 500 W Xe lamp with a 420 nm cutoff filter. An aqueous BiVO4 dispersion was prepared by adding 0.2 g of BiVO4 powder into 100 ml of methyl orange solution (1 mg/L). The solution was magnetically stirred and irradiated by visible-light. After irradiation for a designated time, the dispersion was filtered to separate the BiVO4 particles, and the methyl orange concentration of the filtrate was determined using the Lambda-35 UV-Vis

spectrometer.

RESULTS AND DISCUSSION

The phase and composition of the as prepared products were investigated using XRD measurement (Figure 1). In Figure 1a, all diffraction peaks can be assigned to the monoclinic structure of BiVO4 (JCPDS No. 14-0688) with space Group I2/a and no other peaks for impurities were detected. This observation is further confirmed by the splitting of the peaks at 2θ = 18.5, 35 and 46º, which is characteristic of the monoclinic structure of BiVO4

(Tokunaga et al., 2001). The results show that the monoclinic scheelite BiVO4 could be successfully synthesized by this simple hydrothermal method. Comparatively, for the samples synthesized without adding SDS in the mixed solution, the peaks were more complicated than that of the one with SDS at the same temperature (Figure 1b).There are tetragonal BiOCl phase coexisting with monoclinic BiVO4 in the No SDS samples. The peaks in Figure 1b (marked with stars) fit well with the tetragonal BiOCl (JCPDS No. 85-0861). It suggested that the SDS did improve the crystallization of the BiVO4. It is very helpful to the photocatalytic activity of BiVO4.The morphologies of the monoclinic BiVO4 dendrites prepared by SDS controlling procedures were revealed by SEM (Figure 2). The panoramic view in Figure 2a clearly demonstrates that the as-prepared products are almost entirely dendrite-like crystals with a length of 1 to 2 µm. As shown in Figure 2b, the individual dendrite has well defined branches with uniform diameters of about 100 nm and length of 100 to 500 nm, which demonstrates that the well-defined three-dimensional dendrites are successfully constructed with one dimensional branch components. The morphology and microstructure of monoclinic BiVO4 dendrites were also investigated by transmission electron microscopy (Figure 3). Figure 3a shows a TEM image of part of monoclinic BiVO4 dendrites. It also displays that the dendrites are comprised of 100 nm diameter branches which is consistent with the results of SEM images. Close observation of the samples revealed by HRTEM image (Figure 3b) shows that the surface of the monoclinic BiVO4 dendrites branches is composed of a lot of BiVO4 nano particles with the average diameter of about 8 nm. The clear lattice fringe (Figure 3c) indicates the high-crystallinity and single-crystalline nature of the branches. It can be measured that the d spacing is 0.468 nm, which agrees well with the lattice spacing (011) of monoclinic BiVO4.

In order to investigate the effect of SDS on the morphology of BiVO4 crystals, the samples prepared with the same procedure with different concentration of SDS were obtained. The SEM images (Figure 4) show that the morphologies can be greatly affected by the addition of SDS. Without SDS, the experimental system can only get the rod-like crystals with the diameter of about 100 nm (Figure 4a). After addition of 0.01 M SDS, the nanorods changed into shorter rods (Figure 4b). With 0.02 M SDS, the shorter rods began to aggregate (Figure 4c).

Yin et al. 4289

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

(a)

(b)

2θ(degrees)

Inte

nsity

(a.u

.)

Figure 1. XRD patterns of the BiVO4 synthesized under different conditions ((a) with SDS, (b) without SDS).

(b) (a)

Figure 2. SEM image (a) and enlarged SEM image (b) of monoclinic BiVO4 dendrites.

Continuously increasing the concentration of SDS to 0.03 M resulted in the appearing of small BiVO4 dendrites (Figure 4d). When the concentration was increased into 0.05 M, the products are almost all dendrites BiVO4

(Figure 4e). However, the over high concentration of SDS (0.07 M) can lead to the formation of BiVO4 nanoparticles

(Figure 4f). It can be drown from the results that SDS actually play an important role in controlling the morphologies of BiVO4 dendrites. Combined with the results of XRD measurement (Figure 1b) the rod-like crystals mainly consist of monoclinic BiVO4 and tetragonal BiOCl. This may be due to the formation process

4290 Int. J. Phys. Sci.

0.468 nm (011)

10 nm

(a) (b)

(c)

Figure 3. TEM image (a), HRTEM image (b) and enlarged HRTEM image (c) of monoclinic BiVO4 dendrites.

(a) (b) (c)

(f) (e) (d)

Figure 4. SEM image of BiVO4 prepared with different concentration of SDS ((a) 0 M; (b) 0.01 M; (c) 0.02 M; (d) 0.03 M; (e) 0.05 M, and (f) 0.07 M).

Yin et al. 4291

200 400 600 800 1000 1200

(a)

(b)

Wavelength (nm)

Inte

nsi

ty (

a.u.)

Figure 5. Raman scattering spectra of the monoclinic BiVO4 dendrites (a) and BiVO4 nanorods (b).

process of the monoclinic BiVO4 during which some of BiCl3 can hydrolyzes fast to BiOCl when dissolving in the solvent. Then BiOCl reacts with NH4VO3 to form monoclinic BiVO4. In the SDS absent system, BiOCl can not entirely react with NH4VO3, therefore results in the mixed monoclinic BiVO4 and tetragonal BiOCl phase nanorods. However, when adding SDS into the reaction system, the hydrolyzation of BiCl3 may be inhibited, resulting in the direct reaction of Bi

3+ with NH4VO3 to form

pure monoclinic BiVO4 dendrites. The XRD pattern in Figure 1a with pure monoclinic BiVO4 peaks approved this reaction.

Raman spectra also verified the improved crystallization of monoclinic BiVO4 dendrites. The Raman scattering spectra of the crystalline BiVO4 samples were obtained in different experimental conditions (Figure 5). Raman bands at 330, 365 and 827cm

-1 were observed in

both monoclinic BiVO4 dendrites and BiVO4 nanorods. These Raman bands represented the typical vibration bands of monoclinic scheelite BiVO4 and could be assigned to the asymmetric and symmetric deformation modes of VO4

3- and the symmetric stretching mode of the

V-O bond, respectively (Li et al., 2008). The Raman band at 210 cm

-1 relates to the external mode of BiVO4, which

gives little structural information of the sample (Guo et al., 2010). However, the Raman bands of monoclinic BiVO4 dendrites were slightly different from the BiVO4 nanorods. The peaks of 710 and 640 cm

−1 in Figure 5a, assigned to

the asymmetric V-O1 stretch (as1) and asymmetric V-O2 stretch (as2) disappeared in Figure 5b which hints that the SDS does improve the crystallization of monoclinic

BiVO4 dendrites. The aforementioned results suggested that SDS played a vital role in the formation of monoclinic BiVO4 dendrites. The whole process of SDS affecting the morphology of monoclinic BiVO4 dendrites can be illustrated in Scheme 1. As an anionic surfactant, SDS can form pillar micelles in aqueous solution when the concentration is higher than its critical micelle concentration (CMC) (Step 1). Bi

3+ will produce when

BiCl3 dissolve in the solution and each Bi3+

ion interacts electrostatically with sodium dodecyl sulfate ions to form bi-sodium dodecyl sulfate complexes, which then result in an ordered pillar mesostructure (Step 2). After addition of NH3VO4 the VO

3- can react with Bi

3+ on the pillar

mesostructure surface which then forms the monoclinic BiVO4 nucleus on the surface (Step 3). Continuously reaction of Bi

3+ and VO

3- will result in more monoclinic

BiVO4 nucleus and the congregation of the nucleus along energetically favorable directions on the pillar meso-structure surface (Step 4). These nucleus may eventually developed into the branches of the monoclinic BiVO4 dendrites which can get a hint from the HRTEM of Figure 3b where there are still some undeveloped nucleus on the branch surface. During the hydrothermal process, the aggregated nucleus may undergo an Ostwald ripening process to form larger crystals, that is, the branches of the monoclinic BiVO4 dendrites. Therefore, the well-defined monoclinic BiVO4 dendrites with uniform branch components were successfully obtained by this simple hydrothermal SDS assistant method (Step 5). In order to investigate the effect of SDS on the photocatalytic activities of the as prepared products, the photocatalytic

4292 Int. J. Phys. Sci.

CH3(CH2)11OSO3-

Bi3+

BiVO4 nucleus

Step 1

Step 2

Step 3

Step 4

pillar micelles ordered pillar mesostructure

Electrostatic Interaction Nucleation

Agregation

Ostwald ripening

monoclinic BiVO4 dendrites

Step 5

Scheme 1. Mechanism of the monoclinic BiVO4 dendrites growth affected by SDS.

-20 0 20 40 60 80 100 120 140 160 180 200

0.0

0.5

1.0 (a)

(b)

Degra

dation r

ate

(%

)

Irradiation time (min) Figure 6. Degradation rates of MO on monoclinic BiVO4 dendrites (a) and

BiVO4 nanorods (b) catalysts.

activities of monoclinic BiVO4 dendrites and BiVO4 nanorods were measured in liquid phase reactions. The decomposition of methyl orange in an aqueous solution was chosen as the photoreaction probe. The result shows

that the degradation rates of monoclinic BiVO4 dendrites is much higher than that of BiVO4 nanorods (Figure 6) under the irradiation of visible light (400 nm < λ < 660 nm). Actually, there is almost no reaction when BiVO4

nanorod was used as photocatalyst. This may be due to the fact that the hydrolyzation of BiCl3 into BiOCl decreases the crystallization performance of BiVO4 (Liu et al., 2010). Generally, the particles with well crystallinity can decrease the defects inside the crystals, which allows for the more efficient transfer of electron–hole pairs, generated inside the crystal, to the surface. Therefore, it is not surprising that the well-crystallized monoclinic BiVO4 dendrites show the higher photocatalytic activity than that of BiVO4 nanorod which might also be due to its unique morphologies and higher surface area (the surface area of BiVO4 dendrites is 3.12 m

2/g which is

higher than 1.26 m2/g of BiVO4 nanorods). It was

reported that the photocatalytic property of monoclinic BiVO4 is related to the distortion of the bi-O polyhedron (Zhang et al., 2006). The monoclinic BiVO4 dendrites products are composed of branches with relatively large distortion of the unit cell due to the large surface strain which may be beneficial to its high photocatalytic activities. Conclusion In summary, the monoclinic BiVO4 crystals with dendritic morphology have been synthesized by a facile hydrothermal process. SDS plays a decisive role in the formation of the monoclinic BiVO4 dendrites. It can both enhance the crystallization and determine the morphology of the monoclinic BiVO4 dendrites. In addition, the photocatalytic activities of monoclinic BiVO4 dendrites are also much higher than that of BiVO4 nanorods. These results may not only enrich the dendrite structures of inorganic compounds but also can provide a new surfactant assistant strategy to synthesize hierarchical structures materials. ACKNOWLEDGMENTS We acknowledge the financial support from the Qianjiang Personal Project of Zhejiang Province (no. 2009R10025), Zhejiang Provincial Natural Science Foundation (Y4110499) and the excellent Youth Foundation of the Key Laboratory of Advanced Textile Materials and Manufacturing Technology (Zhejiang Sci-Tech University), Ministry of Education. REFERENCES Abraham F, Debreuille-Gresse MF, Mairesse G, Nowogrocki G (1988).

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